Differences in soil carbon and nitrogen pools between afforested pine forests and natural shrublands in a Mediterranean area

Plant cover and microclimatic conditions can profoundly alter the balance between productivity and decay, with relevant effects on soil carbon (C) and nitrogen (N) pools. Despite their importance, the combined effects of plant cover and microclimatic conditions on soil C and N pools have been poorly assessed, especially in the Mediterranean environment. Therefore, the aims of the present study were to assess how, in Mediterranean areas, C and N pools differ between afforested pine forests and natural shrublands and to highlight the different impacts of plant cover on C and N sequestration at low and high elevations Along a forest floor-mineral soil continuum, plant cover and elevation effects were evaluated through measurements of C and N pools and water-soluble fractions, molecular characterization by 13C and 1H NMR, and microbial and fungal amounts and activities. Our data show that C accumulated more in afforested pine forest soils than in shrubland soils, especially at low elevations. In pine forests, the higher content of aromatic and O alkyl compounds in the upper organic layers and the abundance of aromatic and carboxylic components in the soluble fraction suggest a greater stability of soil organic matter than in shrublands. Additionally, the high concentration of N in the upper organic layer and its reduction in the fermentative layer stimulate soil C accumulation mainly in pine forests at low elevations. The abundant organic mass at high elevations in pine forests improves microbial growth, whereas the greater recalcitrance of organic residues at low elevations in pine forests leads to a decrease in the bacterial component compared with the fungal component. In conclusion, the plant cover effect appears to be strongly conditioned by elevation, and afforestation with pine at low elevations could favour long-term soil C storage.